Large area matrix addressed displays, e.g. of the smectic liquid crystal type, are being introduced in a number of applications. These displays are particularly attractive for their excellent viewing characteristics. In such a display an array of pixels are switched between scattering and clear conditions by the application of suitable voltage waveforms to corresponding row and column conductors. The switching mechanism is described in our United Kingdom Patent Specification No. 1,557,199 (W. A. Crossland - J. Morrissy - B. Needham 10-7-1) and in a paper entitled "Electrically induced scattering textures in smectic-A phases and their electrical reversal" published in the Journal of Physics D: Applied Physics, Volume 11 pages 2025-34.
A disadvantage of present devices resides in the manner in which the row drive waveform is generated. The row waveform comprises a single alternating current cycle of approximately 400 volts peak to peak. In conventional devices this waveform is derived from two direct current power supplies of +200 volts and -200 volts respectively. In consequence the display drive circuitry must be able to withstand an applied voltage difference of about 400 volts. This means that special precautions must be taken and the circuits are consequently costly to manufacture.
The object of the present invention is to minimise or to overcome this disadvantage.